This invention relates to 4 cycle internal combustion engines with adjustable inlet control periods and individual suction pipe segments per cylinder or per inlet.
To obtain high output of such engines without special supercharging units, the objective is to realize as efficient an opening as possible of the inlet valve (large quantity per unit of time). To this end, the inlet valve is opened as early as possible, of course taking into consideration negative effects resulting from the valve overlapping with the closing of the outlet valve; and it is not closed until a maximum cylinder filling has been reached. Another measure to increase the cylinder charge is the use of one inlet line per cylinder, whose dimensions (length and diameter) are coordinated in such a manner that through reflection towards the end of closing the inlet valve overpressure is produced upstream of the inlet valve. The drawback with this arrangement is the required long length of the intake line, which frequently renders the installation difficult in motor vehicles and similar devices.
If such engines are operated over a wide speed range with high torque, they require at full load both at low and high speeds a good cylinder filling and a thermodynamically good combustion. A good filling can be obtained with timing elements with good flow properties, that is with low flow resistance. This applies in particular to the inlet valves. For example, 4 valve engines (2 inlet valves) with channels exhibiting good flow have in this respect at high speeds especially good characteristics. The low flow resistance results in an inflow without any large losses in pressure The cylinder filling is facilitated by the dynamic operations in the suction lines. Due to the reflecting operations during the short inlet periods at high speeds the short lines that are necessitated by the spatial pecularities in engines result in overpressure upstream of the cylinder, before the inlet valves close. In this manner the inflowing fresh charge quantity is enlarged. Simultaneously the high flow rates existing at high speeds ensure an adequately high level of turbulence in the cylinder, which remains largely preserved until the end of the compression also because of the short available time and leads to a thermodynamically good, fast combustion.
In the low speed range, on the other hand, engines have with the known method for inlet control for precisely inlet valves exhibiting good flow both a lower cylinder filling and an unfavorable combustion sequence. First of all, the poor filling is due to the fresh charge being pushed back in part into the intake system owing to the closing of the inlet designed for high speed, as a consequence of the control periods that are usually fixed (in degree crank angle). Second, at low speeds the oscillations in the suction pipe have a poor effect on the cylinder filling, since the wave travel times no longer fit the opening time of the inlet valve. Third, the cylinder charge is heated on the hot walls in the cylinder owing to the long dwell time. In addition, the low rate of inflow at low speed causes a low level of turbulence, which decreases even further as a consequence of the longer residence time until the start of combustion. The result is slow combustion with poor efficiency and increased tendency to knock. In total, the obtainable torque at low speeds is, therefore, unfavorable, a feature that has drawbacks especially for motor vehicle engines and renders tuning the drive difficult for low fuel consumption.
An attempt has already been made to counteract these drawbacks existing at low speeds. For example, suction pipes of variable length can be used, so that at low speed the effective dimension (e.g. length of suction pipe) is adapted to the low speed by changing over or other mechanical means. In so doing, the limited structural space in the motor vehicles often renders the housing of the necessary voluminous dimensions difficult. It is also possible, in particular with several inlet valves to reduce the inflow cross sections by turning off suction pipes acting in parallel, whereby the dynamics and the flow rate can be improved in connection with a length adjustment. The cost for this is relatively high; and the obtainable effects are limited.
Another possibility lies in the rotational adjustment of the inlet camshaft (at low speed counter to the direction of rotation), so that with the same opening duration an earlier closing of the inlet counteracts the return flow following the end of intake at starting piston stroke. Unfortunately, associated with this is an earlier opening of the inlet, resulting in a return flow of the exhaust gas into the inlet channel. This can have an unfavorable effect on the filling and knocking behavior. The requisite turbulence for a rapid knock-free combustion cannot be obtained with all of these measures, if simultaneously the filling loss at maximum output may not become too large.
It is possible with variable opening times of the valves to adjust the control periods to the requirements of an engine over the entire operating range. To date this procedure seeks to change the inlet closure under full load and to change the opening time under partial load. The latter measure serves to reduce the cylinder charge for load control under partial load. However, even with this measure a satisfactory solution to represent a high full load torque at low speeds for high power engines has not been possible to date.